The invention relates to a method for measuring forces exerted on a part in a given direction and in the presence of external pressure applying a force on said part that is large compared with the forces to be measured.
The invention also relates to apparatus for measuring forces by implementing the method.
A privileged application of the invention lies in measuring the compression or traction forces exerted on a member such as a downhole tool or a drill string, e.g. when said member is being put into place in an oil well or when it is being extracted from such a well. The very high pressures that exist at the bottom of an oil well then apply a force on the member in question that is significantly greater than the force which it is desired to measure.
As shown in particular in document U.S. Pat. No. 4,926,396, it is known to measure pressure by using a transducer of tubular shape, made of piezoelectric ceramic. That document also shows that such a transducer, which is normally closed by plates at both ends, responds in different manners to stresses exerted along its axis and to stresses exerted radially, because of external pressure.
In addition, document U.S. Pat. No. 5,107,710 shows that it is known to measure a pressure by means of strain gauges interconnected to form a Wheatstone bridge, and to implant the bridge on a diaphragm, on the side opposite from the pressure that is to be measured.
However, no sensor is known that makes it possible to measure forces exerted in a given direction, independently of an external pressure that is also acting on the sensor and that is capable of giving rise to forces that are significantly higher than the forces to be measured.
Specifically, the invention provides a method of measuring forces F in the presence of an external pressure p, which method is designed to be substantially insensitive to the forces generated by said pressure, including when said forces are much greater than the forces to be measured.
According to the invention, this result is obtained by means of a method of measuring forces F exerted on a part in a first given direction in the presence of external pressure P, the method being characterized in that the deformation of a wall is measured in an axial direction parallel to the first direction and in at least two points of differing wall thicknesses in a second direction forming a non-zero angle with the first direction so as to compensate for the effect of the pressure p.
The invention also provides a method of measuring forces exerted on a part in a given direction and in the presence of an external pressure, the method being characterized in that it consists in integrating a sensor in the part, the sensor being sensitive to the forces F and to the pressure p and delivering an output signal S of the type S=xcex1.F+xcex2.p+xcex3, where xcex1 if a force coefficient, xcex2 is a pressure coefficient, and xcex3 is an offset value, and in dimensioning said sensor in such a manner as to cause the pressure coefficient xcex2 to be zero.
In a preferred embodiment of the invention, a sensor is used comprising a tube including a portion of thinner wall thickness and a portion of thicker wall thickness, having an axis extending along the first direction, and in which deformations along said axis and in a direction orthogonal thereto are measured.
Other embodiments are also possible using sensors including a thinner portion and a thicker portion, e.g. of elliptical shapes or having bulges.
In which case, each wall deformation measurement is performed by two diametrically opposite strain gauges so as to compensate for deformation in bending.
Preferably, under such circumstances, use is made of one pair of strain gauges parallel to the first direction, two pairs of strain gauges orthogonal to the first direction, respectively placed in the thinner portion and in the thicker portion, and one pair of strain gauges that are inactive, the eight strain gauges being interconnected in a Wheatstone bridge, the output signal S from the sensor corresponding to the ratio between an output voltage Vs and an input voltage Ve of said bridge.
Advantageously, when the sensor is fitted with its strain gauges, the pressure coefficient xcex2 can be decreased or even made zero by reducing the outside diameter of one of said tube portions.
In the preferred embodiment of the invention, a tube is used whose thinner portion initially has an inside diameter b0 and an outside diameter a0 defining a first ratio d0=b0/a0 and whose thicker portion initially has an inside diameter B0 and an outside diameter A0 defining a second ratio D0=B0/A0, the first and second ratios being such that:             2      -              ν        0                    1      -              D        0        2              =            1      +              ν        0                    1      -              d        0        2            
where v0 represents the Poisson coefficient as estimated for the tube; variations in the output signal from the sensor as a function of pressure are measured at zero force; the initial pressure coefficient xcex20 of the sensor is deduced therefrom; an optimized value v of the Poisson coefficient of the tube is calculated from the relationship:   ν  =                    2        ⁢                  (                      1            -                          d              0              2                                )                    -              (                  1          -                      D            0            2                          )            +                                                  4              ·              E                        ⁢                          xe2x80x83                                K                ·                  β          0                ·                  (                      1            -                          d              0              2                                )                ·                  (                      1            -                          D              0              2                                )                                    (                  1          -                      d            0            2                          )            +              (                  1          -                      D            0            2                          )            
where E represents Young""s modulus for the tube, and K represents the gauge coefficient; and the pressure coefficient xcex2 is made to be equal to zero:
when xcex20 is negative, by decreasing the outside diameter a0 of the thinner portion to give it a value a such that:   a  =            b      0        ·                  1        -                                            1              +              ν                                      2              -              ν                                ·                      (                          1              -                                                (                                                            B                      0                                                              A                      0                                                        )                                2                                      )                              
and
when xcex20 is positive, by decreasing the initial outside diameter A0 of the thicker portion to give it a value A such that:   A  =            B      0                      1        -                                            2              -              ν                                      1              +              ν                                ·                      (                          1              -                                                (                                                            b                      0                                                              a                      0                                                        )                                2                                      )                              
The invention also provides apparatus for measuring forces F exerted on a part in a given direction in the presence of an external pressure p, the apparatus being characterized in that it comprises a sensor integrated in said part, said sensor being sensitive to the forces F and to the pressure p, and delivering an output signal S of the type S=xcex1.F+xcex2.p+xcex3 where xcex1 represents a force coefficient, xcex2 represents a pressure coefficient, and xcex3 represents an offset value, the sensor being dimensioned in such a manner that the pressure coefficient xcex2 is zero.